Geography Reference
In-Depth Information
Figure 15.3
Immigration and extinction rates determine
the equilibrium number of species that inhabit an
island. Immigration rates vary with proximity to the
mainland. Extinction rates vary with the size of island.
crucial stepping stone in the recolonisation of
other patches. A key difference is that not all
patches need to be occupied all the time by all
species in order to serve a very important
conservation function (Hanski 1996). Some high-
quality habitat areas may act as sources for
emigration to inferior sink areas where mortality
exceeds the birth rate. As a consequence, although
such sink patches may be occupied they may not
necessarily be able to support a viable local
population. Any attempts at species conservation
would need to recognise that many local
populations may not be at equilibrium, and
regional processes may be crucial in sustaining the
metapopulation.
Landscape ecology has developed some indices
of spatial pattern (O'Neill
et al
. 1988) (Figure
15.4). Dominance describes the abundance of a
particular habitat type in the landscape. Contagion
expresses the degree to which habitat fragments
are clustered or dispersed. Fractal dimension
describes the complexity of patch shape. Little
progress has yet been made in exploring the
relationship between these indices and
metapopulation dynamics.
Source:
MacArthur and Wilson 1967.
rates are important in maintaining local
populations, they may not be critical for the
survival of the metapopulation as a whole. The
metapopulation is much more dependent on the
spatial arrangement of habitat fragments. A small
habitat patch, for example, may be too small to
support a viable local population but may be a
Figure 15.4
Indices of spatial structure in landscape
ecology.
Source:
Adapted from O'Neill
et al
. 1988.
Plate 15.1
Regional metapopulation processes may be
crucial in sustaining species populations in habitat
fragments such as small woods in an agricultural
landscape.
